Yes, there are huge national security concerns when everybody's cars are always on the network and all fly-by-wire. It would be trivial for anyone who writes the software for these things to include logic or network bombs in them that would enable a catastrophic strike on a country or the rest of the world. Cars being on the network all the time is really a terrifically stupid idea.

Careful readers may note the complete lack of the word "China" in the previous paragraph. But, yes, technically it is even that much more dangerous to have a whole bunch of potentially-hackable robots roaming up and down all your transportation corridors that were also created by your major economic and military rival.

But I don't even like it when it isn't created by your rival, though, because there's no guarantee that your rival can't hack anybody's software. I fear the day when some significant fraction of cars on the road are given instructions to floor the pedal and then do a hard turn to the left 10 seconds later. (Or the right for countries where that is the "turn into oncoming traffic" direction.)